1. Field of the Invention
The present invention relates generally to interference cancellers, and more specifically to an interference canceller for cancelling wideband frequency interferences from an undesired signal source and adaptive equalization of signal affected by multipath distortions.
2. Description of the Related Art
A prior art interference canceller includes primary and auxiliary antenna systems. As shown in FIG. 1, an input signal from the primary antenna system is applied to a subtractor 1. The primary input contains a desired signal S and a wideband interference component J, both of which are affected by multipath distortions. An input signal J' from the auxiliary antenna is filtered by an adaptive linear transversal filter 2 and subtracted from the primary input by subtractor 1, whose output is coupled to a subtractor 3 where it is subtracted from a training sequence supplied from a reference pulse generator 4. The training sequence is a series of pulses that occur at the same rate as burst signals that appear at periodic intervals in the desired signal. The subtractors 1, 3, the linear filter 2 and the reference pulse generator 4 constitute a circuit known as an adaptive noise canceller and a detailed description of the canceller is given in "Adaptive Signal Processing" by Bernard Widrow and Samual D. Stearns, Prentice-Hall, Inc, 1985.
Signals J and J' are considered as different versions of the same signal transmitted from a common undesired source as they have travelled over different propagation paths and therefore affected by different channel impulse response characteristics. The tap-gain coefficients of linear transversal filter 2 are constantly updated with the output of subtractor 3 according to the least mean square (LMS) algorithm and the filter output is applied to subtractor 1 as an estimate J of the wideband interference J. By subtracting the interference estimate J from the primary input, wideband interference J is cancelled at the output of subtractor 1, allowing the desired signal S to be applied to a decision feedback equalizer comprising a feedforward filter 5, a subtractor 6, a decision circuit 7, an error detector 8 and a feedback transversal filter 9, where it is adaptively equalized to remove intersymbol interference caused by multipath fading.
However, the operation of the prior art interference canceller is satisfactory only when the auxiliary input does not contain a component of the desired signal. If a desired component is contained in the auxiliary input, it would cause an additional multipath distortion or a reduction of the desired signal of the primary input by mutual cancellation. Due to the use of a linear transversal filter for filtering the auxiliary input, receiver noises are distributed along the taps of the linear filter and are mixed with the primary input and the amplification of the noises prevents its tap-gain coefficients from converging to a value sufficient to provide interference cancellation. Additionally, the use of the trailing sequence at burst intervals prevents high speed updating of the tap gain coefficients if the multipath fading of an interference signal varies at a rate higher than the rate at which the training sequence is generated.